Competition among seven Escherichia coli sigma subunits: relative binding affinities to the core RNA polymerase

Seven different species of the RNA polymerase sigma subunit exist in Escher ichia coli, each binding to a single species of the core enzyme and thereby directing transcription of a specific set of genes. To test the a competit ion model in the global regulation of gene transcription, all seven E. coli sigma subunits have been purified and compared for their binding affinitie s to the same core RNA polymerase (E), In the presence of a fixed amount of sigma(70), the principal a for growth-related genes, the level of E sigma( 70) holoenzyme formation increased linearly with the increase in core enzym e level, giving an apparent K-d for the core enzyme of 0.26 nM, Mixed recon stitution experiments in the presence of a fixed amount of cove enzyme and increasing amounts of an equimolar mixture of all seven sigma subunits indi cated that sigma(70) is strongest in terms of core enzyme binding, followed by sigma(N), sigma(F), sigma(E)/ sigma(Fecl), sigma(H) and sigma(s) in dec reasing order. The orders of core binding affinity between sigma(70) and si gma(N) and between sigma(70) and sigma(H) were confirmed by measuring the r eplacement of one core-associated sigma by another sigma subunit, Taken tog ether with the intracellular sigma levels, we tried to estimate the number of each holoenzyme form in growing E. coli cells.